Heretofore, with the object of improving light-emitting diodes in electrostatic breakdown voltage, a large number of methods directed toward improving an LED in breakdown voltage against static electricity by having an electronic part, such as a Zener diode, connected to the LED have been reduced to practice. Particularly, as regards the gallium nitride (GaN)-based LED and the aluminum-gallium-indium phosphide (AlGaInP)-based LED which are formed by using thin compound semiconductor layers, JP-A 2005-20038, for example, discloses an LED that is expected to acquire an enhanced electrostatic breakdown voltage by deliberately having a Zener diode added to and incorporated in the circuit thereof.
Besides, in JP-A 2005-57228 and JP-A 2000-188425, a technique for enhancing the electrostatic breakdown voltage of an LED by causing the LED to be provided with a power supply circuit having capacitors and resistors incorporated in a complicated manner therein is disclosed. The conventional technique, however, necessitates connection of electronic parts, such as a Zener diode and a capacitor to the power supply circuit so as to intend to impart enhanced electrostatic breakdown voltage of the LED. This necessity entails a problem that the LED is unduly enlarged for the sake of securing a space for the installation of the electronic parts. Then, an attempt to increase the number of such electronic parts to be incorporated in the power supply circuit for the purpose of enhancing the electrostatic breakdown voltage to a still better degree entails a problem that the technique for assembling circuits will become complicated and the cost for the fabrication of the LED will inevitably rise.
Besides, a technique for enhancing the breakdown voltage relative to the reverse voltage of an LED by disposing a p-n junction-type LED and a p-n junction-type protective diode separated from the LED and electrically connecting them in a parallel pattern is disclosed in JP-A SHO 52-61982. Also a method which renders an LED difficult to break even on exposure to a reverse voltage by disposing independent p-n junction-type protective diodes adjacently on the same substrate besides following the aforementioned technique of disposing the p-n junction-type protective diode as a simple unit separated from the LED is disclosed in JP-A HEI 10-200159.
The conventional technique which seeks to enhance the reverse voltage of an LED by using a protective diode as a simple component and electrically connecting the relevant components in a parallel pattern as described above, however, necessitates a space for disposing the protective diode and suffers the consequently formed LED to enlarge automatically in chip size. The protective diode disposed as separated from the LED for the purpose of manifesting the function of protecting the LED from the overvoltage in the reverse direction necessitates an electrode for operating the protective diode itself in addition to an electrode for establishing flow of an operating current to the light-emitting part of the LED. The LED that causes the simple p-n junction-type protective diodes disclosed in JP-A SHO 52-61982 to be disposed adjacently, for example, is required to form a total of three (refer to FIG. 4 of the drawings of JP-A SHO 52-61982) or still more a total of four (refer to FIG. 1 of the drawings of JP-A SHO 52-61982) input and output electrodes. This undeniably renders the process for fabricating the LED complicated.
Meanwhile, in the case of the AlGaInP-based compound semiconductor light-emitting device, for example, a method that consists in removing a nontransparent GaAs substrate used in growing an AlGaInP crystal and bonding a transparent GaP substrate with the object of efficiently extracting the emitted light from the device has been developed as disclosed in Japanese Patent No. 3230638 and JP-A 2001-57441.
When the GaP substrate to be used in the light-emitting device of high luminance mentioned above is made to assume the surface of (100) or a surface inclined within 20° from the (100) in the direction of <011>, there ensues a problem that the substrate having a thickness of 100 to 150 μm will suffer from weak mechanical strength and the process of production thereof will incur a decrease in the yield of production due to “cracking.”
If the thickness of the GaP substrate is made to exceed 200 μm with a view to securing mechanical strength, though the decrease of yield due to “cracking” can be diminished, the excess will nevertheless induce such problems as rendering difficult the escape of heat from the interior of the chip, lowering the luminance, and degrading the reliability. The white light-emitting device that is used together with the GaN-based light-emitting device will also suffer the chip height thereof to increase to twice or more the ordinary GaN-based light-emitting device. For this reason, the package thereof together with the GaN-based light-emitting device will incur inconvenience.
As one of the methods available for bonding a transparent substrate such as GaP, for example, a method which effects the bonding through a transparent adhesive material has been developed as disclosed in JP-A 2002-158373.
When the epitaxial layer is exposed as expected by removing a light absorbing substrate which is a grown substrate and further removing partly an LED epitaxial layer as taught in JP-A 2002-158373, however, there ensue such problems concerning assemblage as deteriorating the state of the surface exposed by the etching, degrading the state of the surface of an electrode formed on the surface, and consequently inducing inferior optical recognition. The automatically operated wire bonding device, chip sorter, etc. are required to position pertinent electrodes by optically recognizing them as with a CCD camera. If the electrodes have a coarse surface, the metallic reflection necessary for the optical recognition will fail and this failure will render the recognition inferior. This failure is ascribed to the following reason.
As the substrate to be used for epitaxial growth during the fabrication of an AlGaInP-based compound semiconductor light-emitting device, a substrate having the surface orientation of crystal inclined to the <011> direction from the (100) is used because it is capable of enhancing the crystal quality of an epitaxial growth layer. The common knowledge dictates the theory that the direction of this inclination consists in causing the surface orientation to express the (111)A face from the (100) as proposed in Jpn. J. Appl. Phys., H. Sugawara et al., 31 (1992) 2446. When the surface orientation of crystal of the compound semiconductor possessing such a zinc blende structure as exhibited by GaP or GaAs is to be inclined, the direction of expressing the (111)A face and the direction of expressing the (111)B face are conceivable, depending on the direction in which the crystal axis is inclined from the (100). The (111)A face is the face in which Ga as a Group III element appears predominantly to the surface and the (111)B face is the face in which P or As as a Group V element appears predominantly to the surface. In the substrate having the plane direction of crystal inclined from the (100) toward the <011> direction, the (111)A face constitutes a predominant face on the front surface side and the (111)B face constitutes a predominant face on the back surface side.
When the structure disclosed in JP-A 2002-158373 is adopted, the surface of the epitaxial layer exposed by etching constitutes the back surface relative to the direction of growth. Alternatively mentioned, when the light-emitting part and the ohmic contact epitaxial layer are formed by using a substrate having the surface orientation of crystal inclined from the (100) toward the <011> direction, the (111)B face predominantly constitutes itself the surface of the epitaxial layer to be exposed. Since the (111)B face is characterized by forming a coarse surface in consequence of etching, the exposed surface of the epitaxial layer is inevitably coarsened and is suffered to entail the problem of optical recognition described above.
An object of this invention initiated in view of the problems confronting the conventional techniques as mentioned above is to provide an LED that is enabled to abound in luminance, excel in electrostatic breakdown voltage and assume a small size by causing protecting functions for opposing overvoltage to be contained in the same chip without requiring deliberate attachment of a circuit having electronic parts, such as condensers and resistors, mounted thereon for the sake of improving electrostatic breakdown voltage, a method capable of fabricating the LED inexpensively, and a lamp making use of the LED.
Another object of this invention is to provide an LED that is enabled to attain an excellent electrostatic breakdown voltage by being endowed with a simple structure containing protective functions opposing overvoltage in the interior of a simple LED instead of having such simple electronic parts as protective diodes disposed anew adjacently as separated from the LED and by only fulfilling the necessity for ordinary two electrodes, one positive and the other negative in polarity, necessary at least for operating the LED without adding to the number of electrodes (in other words, number of wire connections), a method of formation capable of fabrocating the LED inexpensively, and a lamp making use of the LED.
Still another object of this invention is to provide an AlGaInP-based light-emitting device that is enabled to abound in luminance and excel in the heat radiating property by lowering the chip height of the light-emitting device till it approximates closely to that of a GaN-based light-emitting device without lowering the yield of process.
Yet another object of this invention is to provide a light-emitting device that is enabled to abound in luminance and promote stabilization of the yield by properly selecting the plane direction of crystal of a substrate for growing a light-emitting layer and consequently allowing amendment of the coarsening caused on the ohmic contact forming surface by an etching process.